1. Field
The present invention relates to the recovery of gold or silver values from an organic liquid or mixed organic/aqueous phase liquids by direct electrowinning of such liquid.
2. Prior Art
Extraction of metal values from aqueous systems by means of ion exchange techniques such as solvent extraction has achieved prominence in hydrometallurgical processing technology.
Ion exchange and solvent extraction are widely used in hydrometallurgy and water purification. Solvent extraction which is carried out by various organic liquid extractants, with or without diluents or modifiers, has been used in hydrometallurgy for the recovery of uranium, copper, and other metals. Synthetic resins which contain functional groups similar to those used in solvent extraction are also widely used for waste water purification and in hydrometallurgy for the separation of metallic ions. The use of resins with active functional groups is a form of extraction that involves a solid substrate. Such a substrate may be, but need not be, active in the ion extraction process. It may function merely as a carrier for an active extractant.
Recovery of metal values from aqueous systems by organic acids, amines, phosphorus oxide compounds, and the like is frequently accomplished from acidic systems. Such extraction is conducted in systems having a pH of less than 7.
Solvent extraction of gold from alkaline solutions has not been practiced heretofore in industry. Laboratory experiments reported in the literature indicate that the extraction of gold from cyanide solutions is possible with weak base amines and neutral extractants such as long chain alcohols, ketones, and the like under acidic conditions.
Extraction of metal values from cyanide systems is generally not practiced industrially inasmuch as cyanide gas may evolve at low pH conditions in which solvent extraction procedures employing amines and the like have been practiced.
In some cases, such as copper, the hydrometallurgical process involves solvent extraction with an organic solution. Subsequently, the metal is stripped from the organic phase into an aqueous phase from which recovery can be achieved frequently by electrolysis.
In the case of gold cyanidation plants, the metal is typically recovered from carbon adsorption eluates by electrowinning from an alkaline solution using steel wool cathodes. Electrical current efficiencies vary from 20% to 50%. The gold which is deposited on steel wool cathodes is impure and has to be further refined by smelting to dorebullion. Also, alkaline cyanide solutions are used in gold electroplating processes which manufacture consumer products. The gold content of these solutions is the major factor which influences the cathodic current efficiency. Low gold concentrations as well as the accumulated impurities in the electroplating solution significantly decrease the current efficiency and the quality of the gold product. From such waste gold electroplating solutions typically an impure gold product is recovered by reduction prior to recycle and/or disposal.
3. Disclosure of Related Applications
A method for extracting gold and silver values from an aqueous system through employment of a solvating extractant under basic conditions has been invented and is the subject of the parent application herein incorporated by reference. In particular, organic oxides of phosphorus and sulfur present as groups pendant from a neutral resinous material are used as extractants for recovering gold and silver from aqueous systems containing gold or silver cyanide anions. Stripping of metal values from the solvating extractant has heretofore been accomplished by temperature adjustment, ionic strength adjustment, or extraction with an appropriate solvent.